The administration of morphine in vivo (3-30 mg/kg) produced a dose-dependent increase in the calcuim-dependent phosphorylation of three synaptosomal cytosolic proteins having molecular weights of 50,000, 55,000 and 60,000. This effect, which could be abolished by pre-treatment with naloxone, but also observed following the administration of levorphanol and 1-methadone. The analgesically inactive stereoisomers, dextrorphan and d-methadone, were ineffective. In rats treated chronically with morphine, phosphorylation patterns were similar to those observed in placebo-treated controls. Morphine was also less effective in stimulating phosphorylation when administered acutely as a challenge does to chronically-treated animals.